The overall objective of the proposed project is to determine the origins of the hippocampal commissures. A combined neurophysiologic and experimental neuroanatomic approach will be used. Horseradish peroxidase will be infused by pressure and by iontophoresis into the terminal fields of CA pyramids and dentate granule cells which will be identified by electrophysiological laminar analysis. After appropriate survival times, brains will be removed and examined histologically in order to identify the commissural cells of origin. These results will be confirmed in a second series of animals by the injection of (3H) leucine/proline into areas which were labelled by HRP in the first series.